1. Field of the Invention
This invention relates to a heat transfer process for the recovery of energy from geothermal brines and other hot water sources, and for desalination. The invention is particularly concerned with a process and system of the above type employing direct contact heat transfer between the hot brine or other hot water feed and an immiscible liquid, or between hot water and such liquid, particularly a liquid hydrocarbon of relatively low vapor pressure, to cause boiling thereof, and effecting direct contact heat transfer between the resulting vapor and water to condense such vapor and heat the water, or brine, employing an arrangement of staged evaporator-condenser units for carrying out such boiling and condensation operations, and utilizing the heated water in an expander or turbine to produce work, or utilizing the heated salt water to generate fresh water.
2. Prior Art
One of the major problems in various approaches suggested for utilization of geothermal energy is the formation of scale on the heat transfer surfaces in contact with geothermal brine. To avoid such problems flash evaporation of brine to generate steam and subsequent use of the steam for power generation or other applications have been proposed. However, in this approach a large drop of temperature will occur due to flashing and a large fraction of the total heat is lost in the brine rejected. If multi-stage flashing is performed, then the pressure of the steam in the latter stages is generally not high enough to drive a turbine.
These approaches are undesirable so far as efficient utilization of geothermal heat is concerned. To circumvent these problems multi-stage flashing of geothermal brine followed by condensation of the steam to transfer heat by indirect heat exchange to a different liquid, usually an organic liquid, has been suggested. However, the main disadvantage in the above approach is that a large surface area is still needed for heat transfer.
U.S. Pat. No. 3,988,895 to Sheinbaum discloses power generation from hot brines by passing such brine through a heat exchanger in direct contact heat exchange relation with a working fluid such as isobutane. The working fluid is vaporized and the vaporized working fluid is passed through a power extracting gas expansion device.
U.S. application Ser. No. 589,068, S. Woinsky, filed June 23, 1975 and U.S. Pat. No. 4,089,175 disclose recovery of energy from geothermal brines by introducing the geothermal brine in a heat transfer zone in direct heat exchange relation with an organic working fluid, the heat transfer zone maintained above, at or below the critical pressure of the working fluid, and expanding the heated working fluid in an expander to produce work.
Desalination is a growing industry in many parts of the world. Not only the countries with vast areas of arid lands, but the developed and the developing countries also are increasingly producing fresh water by desalination to meet the demands of growing population and rising standards of living.
Multi-stage flash and multi-stage evaporation are the most important processes currently in use for desalination of sea water. These processes suffer from two major disadvantages. In the first place, both require large metallic heat transfer surfaces. The cost of the heat transfer surface for these processes is about 35 to 40% of the total capital investment. Also, the corrosion and scaling of these surfaces are difficult to avoid, thus further increasing the cost by the need for replacement of corroded metallic surfaces. Secondly, the cost of energy requirements for these processes is relatively large, of the order of about $2.00 per 1,000 gallons of fresh water produced. Thus, any desalination process which eliminates, partly or wholly, the need for metallic heat transfer surface and/or requires less energy or a lower quality energy has attractive advantages.
A number of other methods and systems are also in use or being developed for desalination. These latter processes are based on the principles of vapor compression, reverse osmosis, freeze crystallization and ion exchange. All of these latter processes are relatively less attractive.
The improved processes described in the Smith U.S. Pat. Nos. 3,640,850 and 3,856,631 are based on heat transfer with direct contact between immiscible fluids. Hence these systems do not require metallic heat transfer surfaces, and such processes can operate with smaller amounts of energy per unit of water produced and with relatively low quality heat.
In the Smith patents hot sea water is flashed in a flash chamber and the water vapor generated is condensed in direct contact with fresh water. The hot fresh water is now brought in contact with a hydrocarbon liquid, which is immiscible with water. The hydrocarbon evaporates and the vapor is condensed in contact with sea water which is heated due to transport of latent heat released from the hydrocarbon vapor, and the heated sea water is heated further by an external heat exchanger. The hot sea water then enters the flash chamber. Although the use of metallic heat transfer surfaces thus is virtually eliminated, this design involves the flow of hydrocarbon and water in opposing directions and in contact with each other, the hydrocarbon following a substantially horizontal flow path between a plurality of evaporator and condenser units.
Other related but less pertinent prior art is set forth below.
The El-Roy patent, U.S. Pat. No. 3,337,421 shows a multi-stage system in which vaporized hydrocarbon is condensed by direct contact with a saline stream. However, the hydrocarbon is vaporized by indirect heat exchange.
Guptill et al in U.S. Pat. No. 3,392,089 discloses liquid-liquid heat exchange between a hot fresh water stream or condensate from a multiple effect evaporator, and a hydrocarbon stream. The heated hydrocarbon is not vaporized as it is chosen to have a high vapor pressure, and is used to transfer heat to a saline stream, by liquid-liquid heat exchange, and the preheated saline stream is fed to the multiple effect evaporator.
U.S. Pat. No. 3,446,711 discloses the condensation of steam by direct contact with a colder liquid hydrocarbon. The heated hydrocarbon is then passed in liquid-liquid exchange with a cold saline feed stream.
Woodward in U.S. Pat. No. 3,219,554 discloses liquid-liquid heat exchange between a hot fresh water stream and a hydrocarbon and between the heated hydrocarbon and an incoming saline stream. The hydrocarbon has a sufficiently high vapor pressure to preclude any vaporization.
U.S. Pat. No. 3,232,847 to Hoff employs a high boiling hydrocarbon which is passed counter current in liquid-liquid exchange to brine in a heating section and is used as a direct contact condensing medium for steam in a second section.
Osdor, U.S. Pat. No. 3,741,878 discloses a similar system in that a low vapor pressure hydrocarbon is used as a heat exchange medium.